The process of identifying a new drug candidate is long and tedious with many promising compounds eliminated from development during preclinical toxicity testing in animals. One reason for the high number of drop out compounds during the preclinical phase is the lack of useful toxicity data early in the discovery program. Many pharmaceutical companies have recognized this in the last several years. The time and expense associated with the drug discovery process has lead to a search for efficiencies that can be realized in the process.
To date, evaluation of in vivo toxicity of a given candidate substance as a potential drug has involved the use of animal models. Underlying the animal tests is the assumption that the effects observed in animals are applicable and predictive of effects in humans. In general, when the dosage is based on a per unit of body surface area, toxicology data from animals is applicable to humans. On the other hand, when the dosage is based on animal body weight, humans are typically more susceptible to toxicity than the test animals. Nevertheless, the vast majority of drugs are developed to be given on the basis of body weight.
Additionally, the actual numbers of animals used in drug testing are much lower than the human population likely to be exposed to the drug if the candidate is actually brought to market. For example, a 0.01% incidence of human exposure to a given drug means that approximately 25,000 out of 250 million individuals are exposed to a drug. To detect such a low incidence in animals would require that 30,000 animals be exposed to the drug. This is clearly an impractical number considering the variety of drugs in development at any given time. Consequently, exposure of fewer animals to high doses of candidate substances is desirable to identify hazards to humans exposed to low doses.
Modern drug development proceeds through a series of stages in which a vast library of compounds is gradually narrowed in a series of successive steps. The use of animals in the initial stages of drug development, in which the number of compounds still being considered is relatively large, is an expensive and inefficient method of producing toxicological data for new drugs especially in light of the fact that most chemicals this early in development, ultimately, Will not be considered drug candidates. Thus, a significant need for alternative toxicological screening methods exists. Indeed, various approaches to toxicological screening prior to the animal testing stage have been proposed.
A common approach to solving the toxicology data deficit has been to incorporate in vitro toxicity testing of compounds of interest into the drug discovery process at a time when new compounds are being identified for potency and efficacy against therapeutic targets. Quality toxicity data at this early stage permits pharmaceutical chemists to attempt to “design out” toxicity while maintaining efficacy/potency. It has proved difficult, however, to develop robust in vitro toxicity testing systems that provide data that is consistently and reliably predictive of in vivo toxicity.
Key issues have been deciding on the type and nature of assays to be utilized and the test system to be employed. There are many biochemical and molecular assays that claim to assess toxicity in cells grown in culture. However, when only one or even two assays are used over a limited range of exposure concentrations, the probability of false negative and false positive data is high. Some of the most commonly used assays include, but are not limited to, leakage of intracellular markers as determined by lactate dehydrogenase (LDH), glutathione S-transferase (GST), and potassium, and the reduction of tetrazolium dyes such as MTT, XTT, Alamar Blue, and INT. All have been used as indicators of cell injury. Prior art in vitro toxicity screens typically only involve the use of one or two endpoints. The resulting data provides a yes/no or live/dead answer. This minimalist approach to the toxicity-screening problem has resulted in little progress towards developing a robust screening system capable of providing a useful toxicity profile that has meaning for predicting similar toxicity in animals. Therefore, there remains a need in the art for the development of new screening systems that provide more useful toxicity information, especially toxicity information that can be obtained rapidly and cost-effectively at early stages of the drug discovery process. A need exists for toxicity screening systems that do not require the use of animals but that provide reliable information on relative toxicity, mechanism of toxicity, and that effectively predict in vivo toxicity.
The drug discovery process is often under significant time pressures, and any time lost while waiting for toxicity data can prove expensive. Thus, a need also exists for in vitro toxicity screening methods and systems optimized for providing relevant information relating to in vitro toxicity in a relatively short time frame.
In some drug development efforts, it is desirable to evaluate the toxicity potential for one or more compounds in particular organ systems. Obtaining this information at a late stage in the process can render significant efforts and expense essentially useless. Thus, a need also exists for in vitro toxicity screening methods and systems that provide relevant information relating to in vivo toxicity in particular organ systems and functions, such as information relating to the cardio toxicity potential of a compound.
Some drug discovery efforts implicate toxicity considerations that are of little or no concern in other efforts. For example, most anti-tumor drugs are either cytostatic or cytotoxic. For cytostatic compounds, off-target toxicity is an important consideration considering the desired result of use of the compound. This consideration is even more critical for cytotoxic compounds. Thus, a need also exists for in vitro toxicity screening methods and systems for specific classes of compounds that have unique or special considerations, such as compounds being investigated for anti-tumor activity.
An important component of any new drug evaluation is the potential for a compound to exhibit species specific toxicity. For example, in the animal testing stage of a drug development effort, rodent studies may show no adverse signs, while a study in a non-rodent species may show severe or even lethal toxicity. When this occurs, repeat animal testing may be required, and significant questions regarding the relevancy of the results to human exposure and toxicity can be raised, each of which can introduce significant delay and expense into the drug discovery effort. Thus, a need also exists for in vitro toxicity screening methods and systems that provide relevant information relating to potential species-specific toxicity.
Another concern during the drug development process is the potential for drug-drug interactions in which one drug alters the pharmacokinetics of a co-administered drug. Having relevant information concerning the ability of a compound of interest to be co-administered with other drugs, or not to be administered with other drugs, may aid in making determinations as to which compounds should be advanced in the process and which compounds should be halted. Thus, a need also exists for in vitro toxicity screening methods and systems that provide relevant information relating to the potential for a compound to produce drug-drug interactions in vivo.
It is to such novel toxicity screening systems that the present invention is directed.